The relative position between an upper leg shaft and the actual prosthetic knee joint is of central importance for the functionality and comfort of the prosthesis fitting. This relative position has a strong influence on the stability of the prosthesis with respect to unintended bending of the prosthesis when standing and on the reaction forces between the upper leg shaft and the limb of the upper leg. Particularly important is aligning the prosthesis in the sagittal plane, which is also described as “setup”.
In every prosthesis setup, a compromise has to be found between adequate stability during standing and the least possible expenditure of force during walking. The greater the stability during standing, the greater the expenditure of force during walking. If the expenditure of force during walking is minimized, an unstable setup of the prosthetic knee joint may occur, which has to be compensated by active use of the hip musculature. This is disadvantageous for the user of the prosthesis.
The stability during standing is achieved in principle by setting back the prosthetic knee joint or the joint axis in relation to the upper leg shaft. This ensures that the weight vector of the prosthesis user originating from the body extends in front of the joint axis of the prosthetic knee joint during standing, whereby the prosthesis remains in the extended position.
During a step, the prosthesis is intended to bend into the swing phase, for which purpose the weight force vector must extend behind the joint axis of the prosthetic knee joint. This is achieved by the prosthesis user introducing a hip moment. As stated above, the physical force to be applied for this purpose depends greatly on the setup of the prosthetic knee joint. If the setup is too stable, initiation in the swing phase involves great expenditure of force. This leads to premature tiring or to pain in the limb of the upper leg.
At present, the positioning of the prosthetic knee joint in relation to the upper leg shaft takes place statically and on the basis of empirical values. Dynamic effects, such as for example the deformation of the system or of the upper leg during walking, are not taken into account.
DE 101 39 333 A1 describes a sensor device and a prosthesis with a sensor device in which the sensor device is arranged in a part of the shinbone below an artificial knee joint. The sensor device provides an outer body, formed as a closed ring, and an inner body, connecting two opposite inner sides of the outer body and having a sensor element for measuring the force acting in the direction of the connecting axis. Ground reaction forces for analyzing walking can be determined by the sensor device.